Changes In Carbon Assimilation Research Articles

Warming-induced increase in carbon uptake is linked to earlier spring phenology in temperate and boreal forests

Under global warming, advances in spring phenology due to rising temperatures have been widely reported. However, the physiological mechanisms underlying the advancement in spring phenology still remain poorly understood. Here, we investigated the effect of temperature during the previous growing season on spring phenology of current year based on the start of season extracted from multiple long-term and large-scale phenological datasets between 1951 and 2018. Our findings indicate that warmer temperatures during previous growing season are linked to earlier spring phenology of current year in temperate and boreal forests. Correspondingly, we observed an earlier spring phenology with the increase in photosynthesis of the previous growing season. These findings suggest that the observed warming-induced earlier spring phenology is driven by increased photosynthetic carbon assimilation in the previous growing season. Therefore, the vital role of warming-induced changes in carbon assimilation should be considered to accurately project spring phenology and carbon cycling in forest ecosystems under future climate warming.

Both climate trends and short‐term fluctuations affected algae–zooplankton interactions in a boreal lake during the late Holocene

Abstract Most studies aiming to explore the response of algae and zooplankton trophic interactions to climate variability have only been focused on unidirectional and very short‐term trends in temperature changes. As a result, the non‐stationary aspect of climate change (warming and cooling periods, frequencies) remains completely unstudied. We studied elemental and stable isotope composition of sedimentary organic matter, photosynthetic pigments, and carbon stable isotope composition of Cladocera resting eggs in a sediment record covering the last c. 2,600 years. We examined how past climate change acting at different timescales affected algal biomass and community composition, and carbon assimilation by zooplankton in a boreal lake. Our study revealed major effects of both long‐term climate trends and shorter‐term fluctuations on algae–zooplankton interactions in a boreal lake. We found the main climate trends, in particular the Little Ice Age, induced algal biomass and community composition changes and drastic changes in carbon assimilation by zooplankton. Interestingly, we found that temperature fluctuations could also contribute to regulating algae–zooplankton interactions. Specifically, we observed drastic changes in sedimentary markers and stable isotope composition of zooplankton remains during the most recent period, suggesting a strong influence of ongoing anthropogenic change on algae–zooplankton interactions. Our study confirms previous findings showing close long‐term linkage between the temporal dynamics of zooplankton diet and planktonic algae, and that both climate trends and short‐term fluctuations are key in regulating consumer–resource trophic interactions. Novel approaches that combine high temporal resolution paleolimnological reconstructions and contemporary monitoring studies are needed to better understand climate change effects on algae–zooplankton interactions and lake food webs.

Physiological and metabolic bases of increased growth in the tomato ethylene-insensitive mutant Never ripe: extending ethylene signaling functions.

The tomato mutant Never ripe(Nr), a loss-of-function for the ethylene receptor SlETR3, shows enhanced growth, associated with increased carbon assimilation and a rewiring of the central metabolism. Compelling evidence has demonstrated the importance of ethylene during tomato fruit development, yet its role on leaf central metabolism and plant growth remains elusive. Here, we performed a detailed characterization of Never ripe (Nr) tomato, a loss-of-function mutant for the ethylene receptor SlETR3, known for its fruits which never ripe. However, besides fruits, the Nr gene is also constitutively expressed in vegetative tissues. Nr mutant showed a growth enhancement during both the vegetative and reproductive stage, without an earlier onset of leaf senescence, with Nr plants exhibiting a higher number of leaves and an increased dry weight of leaves, stems, roots, and fruits. At metabolic level, Nr also plays a significant role with the mutant showing changes in carbon assimilation, carbohydrates turnover, and an exquisite reprogramming of a large number of metabolite levels. Notably, the expression of genes related to ethylene signaling and biosynthesis are not altered in Nr. We assess our results in the context of those previously published for tomato fruits and of current models of ethylene signal transduction, and conclude that ethylene insensitivity mediated by Nr impacts the whole central metabolism at vegetative stage, leading to increased growth rates.

Water relations and non-structural carbohydrate responses to the combined effects of defoliation and progressive drought in a dioecious tree

Increases in drought frequency and insect defoliation are emerging threats that typically affect the plantation production of Populus yunnanensis, which is an endemic dioecious tree species in China. Nevertheless, it is difficult to predict which sex of P. yunnanensis will be more affected by the combined effects of water limitations and defoliation. In this study, we examined the combined effects of progressive drought and defoliation on water relations, growth, gas exchange and leaf carbohydrate reserves in P. yunnanensis females and males to test whether defoliation alters the effects of drought and and how this response varies between genders. The experimental design included two watering regimes (well-watered and drought-stressed) and two levels of defoliation (leaves untouched and 60% defoliated). Defoliation exacerbated drought stress in females by decreasing photosynthesis, dry mass accumulation and leaf non-structural carbohydrate (NSC) concentration. In contrast, the total biomass of males grown under drought combined with defoliation was not significantly altered when compared to those grown under drought alone. These results suggest that defoliation does not aggravate the drought effects on males. The males presented a high capacity to maintain foliar water status through stomatal control under drought conditions irrespective of defoliation treatment. The conservative water use strategy in males resulted in a gradual reduction in leaf water potential, gas exchange rates and NSC. Our data showed that the different responses of the females and males to drought and defoliation were influenced by plant water status and consequent effects on changes in carbon assimilation and leaf non-structural carbohydrate reserves.

Evapotranspiration partitioning for three agro-ecosystems with contrasting moisture conditions: a comparison of an isotope method and a two-source model calculation

Quantification of the contribution of transpiration (T) to evapotranspiration (ET) is a requirement for understanding changes in carbon assimilation and water cycling in a changing environment. So far, few studies have examined seasonal variability of T/ET and compared different ET partitioning methods under natural conditions across diverse agro-ecosystems. In this study, we apply a two-source model to partition ET for three agro-ecosystems (rice, wheat and corn). The model is coupled with a plant physiology scheme for the canopy conductance. The model-estimated T/ET ranges from 0 to 1, with a near continuous increase over time in the early growing season when leaf area index (LAI) is less than 2.5 and then convergence towards a stable value beyond LAI of 2.5. The seasonal change in T/ET can be described well as a function of LAI, implying that LAI is a first-order factor affecting ET partitioning. Application of the model to seven other Ameriflux sites reveals that soil moisture and canopy conductance also influence the ET partitioning. The two-source model results show that the growing-season (May–September for rice, April–June for wheat and June–September for corn) T/ET is 0.50, 0.84 and 0.64, while an isotopic approach shows that T/ET is 0.74, 0.93 and 0.81 for rice, wheat and maize, respectively. The two-source model results are supported by soil lysimeter and eddy covariance measurements made during the same time period for wheat (0.87). Uncertainty analysis suggests that further improvements to the Craig-Gordon model prediction of the evaporation isotope composition and to measurement of the isotopic composition of ET are necessary to achieve accurate flux partitioning at the ecosystem scale using water isotopes as tracers.The program code for the two-source model is available at the open-source platform https://github.com/zhongwangwei/SiLSM_v3.

Host phenology and potential saprotrophism of ectomycorrhizal fungi in the boreal forest

Summary Phenology‐induced changes in carbon assimilation by trees may affect carbon stored in fine roots and as a consequence, alter carbon allocated to ectomycorrhizal fungi. Two competing models exist to explain carbon mobilization by ectomycorrhizal fungi. Under the ‘saprotrophy model’, decreased allocation of carbon may induce saprotrophic behaviour in ectomycorrhizal fungi, resulting in the decomposition of organic matter to mobilize carbon. Alternatively, under the ‘nutrient acquisition model’, decomposition may instead be driven by the acquisition of nutrients locked within soil organic matter compounds, with carbon mobilization a secondary process. We tested whether phenology‐induced shifts in carbon reserves of fine roots of aspen (Populus tremuloides) affect potential activity of four carbon‐compound degrading enzymes, β‐glucuronidase, β‐glucosidase, N‐acetylglucosaminidase and laccase, by ectomycorrhizal fungi. Ectomycorrhizal roots from mature aspen were collected across eight stands in north‐eastern Alberta, Canada, and analysed during tree dormancy, leaf flush, full leaf expansion and leaf abscission. We predicted potential extracellular enzyme activity to be highest when root carbon reserves were lowest, should host phenology induce saprotrophism. Further, we anticipated enzyme activity to be mediated by invertase, a plant‐derived enzyme which makes carbon available to fungal symbionts in the plant–fungus interface. Root carbon reserves were positively correlated with invertase, suggesting phenology may affect carbon allocation to ectomycorrhizal fungi. However, of the four enzymes, host phenology had the largest effect on β‐glucuronidase, but activity of this enzyme was not correlated with root carbon reserves or invertase. Low‐biomass ectomycorrhizas had greater potential laccase activity than high‐biomass ectomycorrhizas, highlighting discrete functional traits in fungi for litter decomposition. Our results suggest that the decomposition of organic matter may be driven by foraging by fungi for nutrients locked within organic compounds rather than for mobilizing carbon. Furthermore, the potential ability to degrade lignin was more common in low‐biomass ectomycorrhizas when compared to high‐biomass ectomycorrhizas. A Lay Summary is available for this article.

Photosynthesis capacity of Quercus petraea (Matt.) saplings is affected by Molinia caerulea (L.) under high irradiance

Molinia caerulea (L.) Moench is a well-known resource competitor to the detriment of tree regeneration in many boreal or temperate forests of the Northern hemisphere. This study aimed to investigate to what extent soil nitrogen uptake in interaction with light availability drives the early (i.e. in two months) establishment of competition between oak (Quercus petraea (Matt.) Liebl.) and M. caerulea saplings. Two-year-old oak was grown in a greenhouse in 20L pots, either alone or in combination with M. caerulea, at two irradiance levels (11% and 55% of incident photosynthetically active radiation, iPAR). Leaf photosynthesis measurements and soil 15N-labeling were used to monitor changes in carbon assimilation and soil nitrogen uptake between and within species under well-watered conditions. Presence of M. caerulea had no significant effect on short-term oak sapling growth regardless of irradiance availability level. However, under 55% iPAR, M. caerulea had higher nitrogen uptake and photosynthesis capacity, whereas sessile oak grown with M. caerulea under 55% iPAR showed the same N uptake and a strong decrease in photosynthesis capacity compared to 11% iPAR. The findings reported here point to a very short-term response of photosynthesis and N uptake in a mixture of M. caerulea – oak saplings under 55% iPAR), which might partly result from greater nitrogen uptake by M. caerulea.

Quantifying the effects of elevated CO2 on water budgets by combining FACE data with an ecohydrological model

ABSTRACTResponse of leaf area index (LAI) is the key determinant for predicting impacts of the elevated CO2 (eCO2) on water budgets. Importance of the changes in functional attributes of vegetation associated with eCO2 for predicting responses of LAI has rarely been addressed. In this study, the WAter Vegetation Energy and Solute (WAVES) model was applied to simulate ecohydrological effects of the eCO2 at two free‐air CO2 enrichment (FACE) experimental sites with contrasting vegetation. One was carried out by the Oak Ridge National Laboratory on the forest (ORNL FACE). The other one was conducted by the University of Minnesota on the grass (BioCON FACE). Results demonstrated that changes in functional attributes of vegetation (including reduction in specific leaf area, changes in carbon assimilation and allocation characteristics) and availability of nutrients are important for reproducing the responses of LAI, transpiration and soil moisture at both sites. Predicted LAI increased slightly at both sites because of fertilization effects of the eCO2. Simulated transpiration decreased 10·5% at ORNL site and 13·8% at BioCON site because of reduction in the stomatal conductance. Predicted evaporation from interception and soil surface increased slightly (<1·0 mm year−1) at both sites because of increased LAI and litter production, and increased soil moisture resulted from reduced transpiration. All components of run‐off were predicted to increase because of significant decrease in transpiration. Simulated mean annual evapotranspiration decreased about 8·7% and 10·8%, and mean annual run‐off increased about 11·1% (59·3 mm year−1) and 9·5% (37·6 mm year−1) at the ORNL and BioCON FACE sites, respectively. Copyright © 2014 John Wiley & Sons, Ltd.

Radioactive Chernobyl Environment Has Produced High-Oil Flax Seeds That Show Proteome Alterations Related to Carbon Metabolism during Seed Development

Starting in 2007, we have grown soybean (Glycine max [L.] Merr. variety Soniachna) and flax (Linum usitatissimum, L. variety Kyivskyi) in the radio-contaminated Chernobyl area and analyzed the seed proteomes. In the second-generation flax seeds, we detected a 12% increase in oil content. To characterize the bases for this increase, seed development has been studied. Flax seeds were harvested in biological triplicate at 2, 4, and 6 weeks after flowering and at maturity from plants grown in nonradioactive and radio-contaminated plots in the Chernobyl area for two generations. Quantitative proteomic analyses based on 2-D gel electrophoresis (2-DE) allowed us to establish developmental profiles for 199 2-DE spots in both plots, out of which 79 were reliably identified by tandem mass spectrometry. The data suggest a statistically significant increased abundance of proteins associated with pyruvate biosynthesis via cytoplasmic glycolysis, L-malate decarboxylation, isocitrate dehydrogenation, and ethanol oxidation to acetaldehyde in early stages of seed development. This was followed by statistically significant increased abundance of ketoacyl-[acylcarrier protein] synthase I related to condensation of malonyl-ACP with elongating fatty acid chains. On the basis of these and previous data, we propose a preliminary model for plant adaptation to growth in a radio-contaminated environment. One aspect of the model suggests that changes in carbon assimilation and fatty acid biosynthesis are an integral part of plant adaptation.

Assessment of six Indian cultivars of mung bean against ozone by using foliar injury index and changes in carbon assimilation, gas exchange, chlorophyll fluorescence and photosynthetic pigments

Six Indian cultivars of Vigna radiata L. (HUM-1, HUM-2, HUM-6, HUM-23, HUM-24 and HUM-26) were exposed with ambient and elevated (ambient + 10 ppb ozone (O3) for 6 h day(-1)) level of O3 in open top chambers. Ozone sensitivity was assessed by recording the magnitude of foliar visible injury and changes in various physiological parameters. All the six cultivars showed visible foliar symptoms due to O3, ranging 7.4 to 55.7 % injured leaf area. O3 significantly depressed total chlorophyll, photosynthetic rate (Ps), quantum yield (F(v)/F(m)) and total biomass although the extent of variation was cultivar specific. Cultivar HUM-1 showed maximum reduction in Ps and stomatal conductance. The fluorescence parameters also indicated maximum damage to PSII reaction centres of HUM-1. Injury percentage, chlorophyll loss, Ps, F(v)/F(m) and total biomass reduced least in HUM-23 depicting highest O3 resistance (R%).

Herbivore induction of jasmonic acid and chemical defences reduce photosynthesis in Nicotiana attenuata

Herbivory initiates a shift in plant metabolism from growth to defence that may reduce fitness in the absence of further herbivory. However, the defence-induced changes in carbon assimilation that precede this reallocation in resources remain largely undetermined. This study characterized the response of photosynthesis to herbivore induction of jasmonic acid (JA)-related defences in Nicotiana attenuata to increase understanding of these mechanisms. It was hypothesized that JA-induced defences would immediately reduce the component processes of photosynthesis upon attack and was predicted that wild-type plants would suffer greater reductions in photosynthesis than plants lacking JA-induced defences. Gas exchange, chlorophyll fluorescence, and thermal spatial patterns were measured together with the production of defence-related metabolites after attack and through recovery. Herbivore damage immediately reduced electron transport and gas exchange in wild-type plants, and gas exchange remained suppressed for several days after attack. The sustained reductions in gas exchange occurred concurrently with increased defence metabolites in wild-type plants, whereas plants lacking JA-induced defences suffered minimal suppression in photosynthesis and no increase in defence metabolite production. This suppression in photosynthesis occurred only after sustained defence signalling and defence chemical mobilization, whereas a short bout of feeding damage only transiently altered components of photosynthesis. It was identified that lipoxygenase signalling interacted with photosynthetic electron transport and that the resulting JA-related metabolites reduced photosynthesis. These data represent a metabolic cost to mounting a chemical defence against herbivory and link defence-signalling networks to the differential effects of herbivory on photosynthesis in remaining leaf tissues in a time-dependent manner.

Carbon isotopic composition of cypress trees from South Florida and changing hydrologic conditions

δ 13C values were determined from cypress tree rings from two different study areas in South Florida. One site is located in the Southeastern Everglades Marsh, where pond cypress ( Taxodium ascendens) was sampled from tree islands (annual tree rings from 1970 to 2000). Bald cypress ( Taxodium distichum) trees were sampled at the other site, located along the Loxahatchee River in a coastal wetland (decadal tree rings from 1830 to 1990). The isotopic time series from both sites display different, location-specific information. The pond cypress isotopic time series has a positive correlation with the total amount of annual precipitation, while the bald cypress data from the Loxahatchee River study area had two different records dependent on the level of saltwater stress. In general, for terrestrial trees growing in a temperate environment, water stress causes an increase in water-use efficiency (WUE) resulting in a relative 13C enrichment. Yet, trees growing in wetland settings in some cases do not respond in the same manner. We propose a conceptual model based on changes in carbon assimilation and isotopic fractionation as controlled by differences in stomatal resistance (water stress) and mesophyll resistance (biochemical and nutrient related) to explain the isotopic records from both sites. With further work and a longer time series, our approach may be tested, and used to reconstruct change in hydroperiods further back in time, and potentially provide a baseline for wetland restoration.

Effects of soil water deficit and nitrogen nutrition on water relations and photosynthesis of pot-grown Coffea canephora Pierre

Coffea canephora plants (clone INCAPER-99) were submitted to low N (LN) or high N (HN) applications and two watering regimes (daily irrigation and irrigation every 5 days for a month). Although water potential was not altered significantly by N, HN plants showed higher relative water content than did LN plants under water deficit. Only HN plants exhibited some ability for osmotic adjustment. Plants from both N treatments increased their cell wall rigidity under drought, with a more pronounced augmentation in HN plants. In well-watered plants, carbon assimilation rate increased with increasing N while stomatal conductance did not respond to N supply. Under drought conditions, carbon assimilation decreased by 68–80% compared to well-watered plants, whereas stomatal conductance and transpiration rate declined by 35% irrespective of the N applications. Stable carbon isotope analysis, combined with leaf gas exchange measurements, indicated that regardless of the watering treatments, N increased the long-term water use efficiency through changes in carbon assimilation with little or no effect on stomatal behaviour.

Responses of sugar beet roots to iron deficiency. Changes in carbon assimilation and oxygen use.

Different root parts with or without increased iron-reducing activities have been studied in iron-deficient and iron-sufficient control sugar beet (Beta vulgaris L. Monohil hybrid). The distal root parts of iron-deficient plants, 0 to 5 mm from the root apex, were capable to reduce Fe(III)-chelates and contained concentrations of flavins near 700 microM, two characteristics absent in the 5 to 10 mm sections of iron-deficient plants and the whole root of iron-sufficient plants. Flavin-containing root tips had large pools of carboxylic acids and high activities of enzymes involved in organic acid metabolism. In iron-deficient yellow root tips there was a large increase in carbon fixation associated to an increase in phosphoenolpyruvate carboxylase activity. Part of this carbon was used, through an increase in mitochondrial activity, to increase the capacity to produce reducing power, whereas another part was exported via xylem. Root respiration was increased by iron deficiency. In sugar beet iron-deficient roots flavins would provide a suitable link between the increased capacity to produce reduced nucleotides and the plasma membrane associated ferric chelate reductase enzyme(s). Iron-deficient roots had a large oxygen consumption rate in the presence of cyanide and hydroxisalycilic acid, suggesting that the ferric chelate reductase enzyme is able to reduce oxygen in the absence of Fe(III)-chelates.

Effects of soil drought during the vegetative phase of seedling growth on the uptake of 14co2 and the accumulation and translocation of 14C in cultivars of field bean (vicia faba L. Var. Minor) and field pea (pisum sativum L.) Of different drought tolerance

During the vegetative phase of growth of two field bean and two field pea cultivars of different drought tolerance, the effect of short and prolonged soil drought on gas exchange (CO2 i H2O), leaf water potential (ψ), stomatal diffusive resistance (rS), uptake of CO2, and the distribution and accumulation of 14C was studied. Differences in the response to drought conditions between resistant and susceptible cultivars were marked. After 5 days of soil drought, the decrease in net photosynthesis and transpiration rate and the increase of stomatal resistance were greater in the drought‐resistant cultivars than in the drought‐susceptible ones. In contrast, after 10 days of drought the decrease of leaf PN (CO2 assimilation rate), E (rate of transpiration) and ψ (water potential) was greater in the susceptible cultivars than in the resistant ones. Significant differences between the resistant and the susceptible cultivars were also observed in the assimilation and translocation of 14C by the green parts of the plant. The amount of carbon accumulation in roots in drought‐susceptible cultivars increased less than in the drought‐resistant cultivars. For treatments in which optimal soil watering was resumed after 5 or 10 days of drought there was no evidence of effects of drought on the majority of measurements, but the drought‐resistant cultivars showed a general tendency for a more rapid recovery. Our results confirm the existence of genetic variability in drought tolerance among the cultivars of field bean and field pea. The recorded differences in the response to drought of experimental cultivars may indicate that, under water deficit in the soil and in plant tissues, they may use different strategies to avoid the damaging effects of temporary limitation of water supply; for example, the drought‐resistant cultivars may more effectively conserve tissue hydration through effective stomatal closure. Also, the observed changes in carbon assimilation and accumulation might be the reason for their different responses to drought. The change in radioactivity losses in the control and stressed plants may result from the differences in demand for energy to maintain cell structure and function. Similarly, the less intense carbon accumulation in the roots of the sensitive cultivars could be caused by more harmful effects of drought on root growth.

Drought effects on photosynthesis of the mangrove, Avicennia germinans, under contrasting salinities

Drought effects on leaf photosynthesis of A. germinans growing under two contrasting salinities were studied in a Venezuelan fringe mangrove. During both wet and dry seasons, severe chronic-photoinhibition at predawn was not observed but strong down regulation occurred at midday during both seasons. Carbon assimilation rates (A, μmol CO2 m−2 s−1) declined during the dry season from 11.9±1.8 to 7.0±1.5 and from 9.6±2.0 to 4.7±2.5 in plants from low and high salinity sites, respectively. Changes in carbon assimilation per unit of chlorophyll (A/Chl, mmol CO2 mol−1 Chl) were from 31.6±4.7 to 20.5±4.3 and from 21.9±4.7 to 15.2±8.2 in the low and high salinity plants, respectively. Therefore, neither changes in Chl nor seasonal differences in photoprotective down regulation could account fully for the decrease in leaf photosynthesis during drought. A reduction in CO2 diffusion due to lowered stomatal conductance was not large enough to explain such a dramatic effect of drought on leaf photosynthesis. Stomatal response could be mitigated by the capability of A. germinans for osmotic adjustment under high salinity and/or drought. However, this intracellular salt accumulation may reduce carbon assimilation capacity further by decreasing the metabolism of leaf cells, increasing dark respiration and/or photorespiration.

DIFFERENTIAL SENSITIVITY OF DUCKWEEDS (LEMNACEAE) TO SULPHITE

SummaryFrond replication and some aspects of carbon assimilation were observed in three species of duckweed (Lemnaceae) grown at two levels of irradiance in the presence and absence of sulphite. The objectives were to examine the relationship between the inhibition of frond replication caused by sulphite, and changes in carbon assimilation, and the role of irradiance in modifying carbon assimilation and sensitivity to sulphite.Under low irradiance (40μ.mol m−2 s−1 photosynthetic photon flux density (PPFD)), inhibition of frond replication by sulphite was most pronounced in Lemna gibba, producing a slower frond doubling time and reduced frond fresh weight. Spirodela oligorhiza exhibited a slower doubling time only, whereas neither parameter was affected in Lemna valdiviana. In all species, chlorophyll content increased in response to sulphite enrichment. Soluble protein levels increased when frond replication was significantly inhibited in L. gibba and S. oligorhiza. Photosynthesis and dark respiration rates were reduced in all species by 10 mM sulphite while ribulose bisphosphate carboxylase activity on a soluble protein basis was increased up to 11%. Reduced carbon assimilation may have contributed to decreased frond replication in L. gibba and S. oligorhiza.In all species, photosynthesis and ribulose bisphosphate carboxylase activity were enhanced under high irradiance (200 μmol m−2 s−1 PPFD). These physiological responses to elevated light appear to have contributed to faster rates of frond replication in all three species, and reduced sensitivity of frond replication to sulphite in L. gibba and S. oligorhiza. It is postulated that the stimulation of carbon assimilation under high irradiance is an important physiological response contributing to the modification by the photoenvironment of the sensitivity of duckweeds to sulphite.

Photosynthetic Assimilation of14CO2by Waterstressed Sunflower Leaves at Two O2Concentrations and the Specific Activity of Products

Photosynthetic Assimilation of¹⁴CO₂by Waterstressed Sunflower Leaves at Two O₂Concentrations and the Specific Activity of Products